This invention relates to the field of composite structural materials and is particularly directed to metal aluminide fiber reinforced composite materials.
Performance requirement goals for future advanced airframe structures and gas turbine engines exceed the capabilities and limits of currently available materials and manufacturing technologies. Improvements in lightweight, high temperature materials and processes are required to meet the challenging goals. Metal aluminides, particularly titanium aluminide base alloys, offer opportunities for weight reduction compared to nickel base superalloys. To achieve the ambitious high temperature capability goal in a light and stiff material, it has been proposed to fabricate fiber reinforced composites using titanium aluminide base alloys as the matrix. However, as high strength and high temperature matrix materials are selected to provide high performance composites, it becomes more difficult to fabricate the composites because the temperatures and pressures required to consolidate the materials also increase.
Composites can be fabricated by placing a reinforcing material, such as silicon carbide fibers, between layers of a matrix material, such as a metal alloy. These ingredients are then consolidated into a composite by pressing them together at a temperature and pressure which will cause the matrix to flow around the reinforcing fibers and diffusion bond the matrix together.
At present, there is a great deal of interest in consolidating fiber composites with high temperature metallic matrices, such as titanium and nickel intermetallics (Ti.sub.3 Al, TiAl, Ni.sub.3 Al, NiAl). These alloys are being produced currently with several alloying additions in powder form via rapid solidification processing (RSP) technology. While the structural properties of the RSP powder alloys are desirable as matrix, powder consolidation directly with reinforcing fibers, e.g., SiC, causes several difficulties. At lower consolidation temperatures, the hard metallic particles can crack or mechanically damage fibers during hot pressing and hot isostatic pressing operations. At high consolidation temperatures, the reactive powder alloys can chemically react and degrade fibers. An additional problem is presented that if the powder particles surrounding the fibers are not well bonded or are too brittle, during cooldown from the fabrication temperature or subsequent thermal cycling in service, tensile stresses induced in the matrix can cause cracking of the matrix, e.g., typical mid-plane cracking between fibers.
Thus, in summary, the problems presented are (1) interfacial reaction between the metal alloy particles and the fiber materials leading to brittle reaction products formed near the interface, (2) there is a CTE (coefficient of thermal expansion) difference between the metal alloy and the reinforcing fibers which causes large tensile stresses in the metal alloy matrix, thereby tending to cause cracks, and (3) the metal matrix itself, that is, the metal aluminide, is a relatively brittle material and tends to crack easily.
In Applicant's copending U.S. application Ser. No. 182,676, filed Apr. 18, 1988, now U.S. Pat. No. 4,847,044, titled "A Method of Fabricating a Metal Aluminide Composite" and assigned to the same Assignee as the present application, there is disclosed adding to a metal aluminide composite during fabrication a soft metal phase, such as aluminum, or a metal forming a metal aluminide, or an alloy containing these metals, to promote consolidation of the metal aluminide matrix with the reinforcing phase. The softer metal, the metal aluminide matrix, e.g., titanium aluminide, and the reinforcing phase are pressed together at a temperature above the softening temperature of the softer metal. The softened metal promotes flow and consolidation of the matrix and the reinforcement at relatively low temperatures. The composite is held at an elevated temperature to diffuse and convert the soft metal phase into the metal aluminide matrix.
It is an object of the invention to provide a method of fabricating a metal aluminide composite by consolidation of a metal aluminide alloy and fibrous reinforcing material under conditions to produce a metal aluminide matrix composite having improved structural properties.
Another object is the provision of a method of fabricating a metal aluminide matrix composite without damaging the reinforcing fibrous material during hot pressing operations.
A further object of the invention is to provide a method of fabricating a metal aluminide matrix while minimizing or avoiding chemical reaction between the metal aluminide and the fibrous reinforcing material, to thereby avoid degradation of the fibers.
Yet another object of the invention is the provision of procedure for fabricating a metal aluminide matrix composite while reducing tensile stresses induced in the matrix during cooldown and subsequent thermal cycling, and avoiding cracking of the metal aluminide matrix under such conditions.
A still further object of the invention is to provide a method of fabricating a metal aluminide matrix composite formed between a metal aluminide matrix and a fibrous reinforcing material while enhancing the ductility of the matrix alloy.
Yet a further object of the invention is the provision of a method of fabricating a metal aluminide matrix composite by consolidation of a metal aluminide alloy, such as titanium aluminide, and a reinforcing fibrous material, such as silicon carbide fibers, by the addition of an element which achieves the aforementioned objects, and which also particuarly reduces the CTE mismatch between the metal aluminide matrix and the reinforcement fibers, thus minimizing crack formation in the metal matrix.